The term comes from the word "Landlord", and it refers to a person, who is the lord/boss/owner of one or several bitches. - A bitch here is not the formal term of the female dog, but a bitch person.
Other, less significant synonyms: Bitch owner, Bitch boss, and the Spanish version - Amo Puta - is also widely spread.

-Hey dude, are you coming out tonight?
-No, I have to go pleasure my Bitchlord.
-You're such a bitch!

Do you remember Anita from the party last week? I made her my bitch. Now I'm officially a Bitchlord.